(A) C 3 H 8 (g) + O 2 (g) ® CO 2 (g) + H 2 O (g) (b) C 3 H 8 (g) + O 2 (g) ® CO

Question

(A)       C3H8 (g) + O2 (g) ® CO2 (g) + H2O (g)

(b) C3H8 (g) + O2 (g) ® CO2 (g) + CO (g) + H2O (g)

The equation presented above shows (a) combustion of propane with abundant oxygen and (b) incomplete combustion of propane when oxygen availability is limited. Use these equations to answer the following questions.

(a) Balance both equations

(b) How much carbon dioxide (g) will be produced after burning a 20-pound propane tank?

(c) How much carbon monoxide (g) will be produced after burning a 20-pound propane tank with a limited access to oxygen (incomplete combustion)?

(d) Calculate the heat released (enthalpy of reaction) for each reaction if an entire 20-pound propane tank is used?

Explanation / Answer

C3H8 + O2 CO2 + H2O
To balance the C’s, make 3 CO2.
C3H8 + O2 3 CO2 + H2O
To balance the H’s, make 4 H2O.
C3H8 + O2 3 CO2 + 4 H2O
To balance the O’s, make 5 O2.
C3H8 + 5 O2 3 CO2 + 4 H2O

The coefficients in a balanced equation determine the ratio of moles of reactants to mole of products. In this equation, 1 mole of C3H8 reacts with 5 moles of O2 to produce 3 moles of CO2 and 4 moles of H2O. Since the ratio of moles of C3H8 moles of CO2 is 1: 3, let’s use the following equation to determine the number of moles of CO2.

1/3 = Moles of C3H8 / Moles of CO2

If we assume that the pressure and temperature are standard, the volume of one mole is 22.4 liters. To determine the number of moles, use the following equation.

Number of moles = Mass ÷ Mass of 1 mole
Let’s determine the mass of 1 mole of C3H8 and CO2
For C3H8, mass = 3 * 12 + 8 = 44 grams
For CO2, mass = 12 + 2 * 16 = 44 grams

Let’s convert 20 pounds to grams.
Mass = 20 lb * 454 g/lb = 9080 grams
Mass of 1 mole = 3 * 12 + 8 = 44 grams
Number of moles = 9080 ÷ 44

1/3 = 9080 ÷ 44/ Moles of CO2
Moles of CO2 = 3 * 9080 ÷ 44
This is approximately 619 moles. This is the number of moles of CO2. To determine the mass, multiply by 44.

Mass = 44 * 3 * 9080 ÷ 44 = 27,240 grams

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